Question 48
Real numbers $x, y \in (0, \pi)$ satisfy the equality
$$
\cos 2x \cos y - \cos 2y \cos x = \cos y - \cos x.
$$
Show that $x = y$.
$$
\cos 2x \cos y - \cos 2y \cos x = \cos y - \cos x.
$$
Show that $x = y$.
MathNet 2012 Question 48
Math Olympiad Vault · MathNet 2012